Magnetic-drive centrifugal pumps may be classified as synchronous or nonsynchronous. Synchronous pumps generally use magnetic coupling between a first magnetic cylinder and a second magnetic cylinder, which are separated by a containment shell. The first magnetic cylinder is coaxially oriented with respect to the second magnetic cylinder. Nonsynchronous drive centrifugal pumps use eddy current coupling between a magnetic cylinder and a torque ring, which is typically made of steel and copper. For nonsynchronous magnetic-drive pumps, the magnetic cylinder is coaxially oriented with respect to the torque ring.
Background art magnetic-drive centrifugal pumps frequently use journal or sleeve-type bearings in conjunction with a pump shaft. For optimum operation, sleeve-type bearings require the formation of a thin lubricating film between the shaft and the bearing surfaces. Whether or not the requisite lubricating film is formed between the shaft and the bearing surfaces may depend upon the viscosity of the lubricant, the rotational speed of the surfaces, and the load pressure applied to the surfaces.
There are two basic lubrication operating regimes for the shaft and bearing surfaces: (1) a hydrodynamic film lubrication regime, in which wear of the surfaces is minimal or nonexistent, and (2) a mixed film lubrication regime, in which wear of the surfaces occurs. In the hydrodynamic film lubrication regime a thin lubricating film is present and prevents the shaft journal from directly contacting the bearing surface. On the other hand, in the mixed film lubrication regime a journal is partially supported by a thin lubricating film and partially supported by direct rubbing contact between the wearing surfaces.
Background art journal bearings and sleeve-type bearings may have axial, curved, helical, or spiral grooves to improve distribution of the lubricant on bearing and shaft surfaces. However, a groove in a journal bearing invariably results in increased wear of an associated shaft journal or decreased radial loading capacity of an associated shaft. A grooved bearing has less surface area for supporting a given radial load than a conventional bearing without a groove. Therefore, a grooved bearing typically operates in the mixed film lubrication regime. If, for example, a grooved journal bearing is used in conjunction with a stationary shaft, than the groove in the journal bearing rotates in alignment with the radial load force vector upon each revolution of the bearing. In general, when the groove is in alignment with the radial load force vector, then the shaft and the bearing surfaces operate in the mixed film lubrication regime. Hence, the mixed film lubrication causes the shaft or bearing surfaces to wear and may damage the shaft or bearing surfaces. Spiral or curved grooves, in the cylindrical interior surfaces of journal bearings, are difficult to machine when the bearings are constructed from hardened metals or ceramics.
Magnetic-drive centrifugal pumps typically use sleeve-type or journal bearings that are lubricated by the pumped fluid. For example, FIG. 1 discloses a prior art bearing 10 and a prior art shaft 24, which are product-lubricated. The prior art shaft 24 has a substantially cylindrical shaft surface 28 with an optional threaded segment 32 for affixing the shaft 24 to a vaned rotor. The prior art shaft 24 also has a head 26 to facilitate attachment to a rotatable magnetic cylinder or a torque ring. The prior art bearing 10 includes a sleeve 14 with a cylindrical inner surface 16. The cylindrical inner surface 16 comprises an appropriate product lubricated wearing surface. The prior art bearing 10 has bypass holes 20 axially extending through the bearing 10. In addition, a face of the bearing 10 preferably has radial notches 18.
FIG. 2 illustrates the prior art bearing 10 and the prior art shaft 24 incorporated into a magnetic-drive centrifugal pump wherein the prior art shaft 24 is rotatable. The prior art bearing 10 is secured to the casing 54 by bearing holder 50. The prior art shaft 24 is affixed to the vaned rotor 36 and is secured by a shaft nut 34. The prior art shaft 24 is also attached to a torque ring 44. The vaned rotor 36 is bounded by a front wear ring 38 and rear wear ring 40. Similarly, the prior art bearing 10 is bounded by a front thrust washer 46 and a rear thrust washer 48. A containment barrier 42 is secured to the casing 54. The casing 54 has an internal channel 52 extending from the periphery of the rotor 36 to the torque ring 44.
While the majority of the fluid is conveyed from the casing inlet 56 to the casing outlet 58, a minority of the fluid is circulated via the channel 52 of the pump to provide lubrication and cooling of the prior art shaft 24 and the prior art bearing 10. A main fluid flow path 60, and a internal circulation path, which includes a bushing flow path 62 and a bypass flow path 64 are shown in FIG. 2 as dotted lines with arrows indicating the direction of flow. The main fluid flow path 60 extends from the casing inlet 56 to the casing outlet 58. The internal circulation path starts near the rotor's discharge at a starting point 68. From the starting point 68 the internal circulation fluid path contacts the wearing surfaces of the bearing 10 and the shaft 24 via the bushing flow path 62. The internal circulation path includes the bypass flow path 64 through the bypass holes 20. Finally, the internal circulation path ends near the eye of the vaned rotor 36 at a termination point 72. The internal circulation occurs because of the pressure-velocity differential between the starting point 68 and the termination point 72. Specifically, the starting point 68 is near the high pressure of the casing outlet 58 and the termination point 72 is near the suction of the vaned rotor 36.
In the prior art pump shown in FIG. 2, the bypass holes 20 may pass liquid while gases and vapor accumulate at the interface between the bearing 10 and the shaft 24. Therefore, the product-lubricated wearing surfaces of the bearing 10 and shaft 24 are exposed to intervals of diminished lubrication because of the presence of the vapor component of the pumped fluid. The selective and problematic routing of vapor and gases to the wearing surfaces is caused by the centrifugal action fluid near the bearing 10.
In general, another problem in background art centrifugal pumps is that particulate matter, or solid particles, in the pumped fluid become trapped in the space between the bearing surface and the pump shaft. Trapped particles may scratch or score either the bearing or the shaft surfaces causing premature bearing or bearing failure. Some particles may even adhere to the shaft or bearing surfaces; especially after the shaft surfaces have been scratched or scored. Moreover, trapped particles lodged in the internal circulation path may further impede the cooling of the shaft. Cooling may be impeded because of increased hydraulic resistance of the congested bushing flow path. In addition, thermal problems may arise from particles adhering to bearing or shaft surfaces and preventing pumped fluid (i.e. lubricant) from contacting these surfaces.
Thus, a need exists for an improved shaft for a magnetic-drive centrifugal pump. In particular, a need exists for a shaft which reduces or ameliorates thermal problems and shaft scoring associated with trapped particles at the pump shaft-bearing interface. Moreover, a need exists for a shaft and bearing combination having increased longevity compared to prior art shaft and bearing combinations.